.Net: Updates to FileIOPlugin

[markwallace-microsoft]

Motivation and Context

1. AllowedFolders - Provide control over which folders can be written to
2. DisableFileOverwrite - Disable overwriting existing files

Description

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[markwallace-microsoft]

Merge integration tests, one test failed

[xUnit.net 00:00:30.20]     SemanticKernel.IntegrationTests.Connectors.AzureOpenAI.AzureOpenAITextToImageTests.ItCanReturnImageUrlAsync [FAIL]
[xUnit.net 00:00:30.20]       Microsoft.SemanticKernel.HttpOperationException : HTTP 429 (: 429)
[xUnit.net 00:00:30.20]       
[xUnit.net 00:00:30.20]       Rate limit is exceeded. Try again in 8 seconds.